I was re-reading the novel, and after looking at a map of Westeros, I was just wondering how the Iron Fleet could reach and capture the Shield Islands without being seen or being obstructed by the Lannister fleet in Lannisport. Would they have sailed far, far away from the coast to avoid this? And couldn't the Lannister fleet to try and recapture the Shield Islands themselves? I know they would need more ships, but geographically speaking, the islands don't seem that far for them to do so.

It is explained in the book. They say that Euron had the idea to sail out into the sea, then sweep back in toward the shield islands. This was specifically done to avoid being seen traveling south along the coast by Lannisport or whoever else.

It is explained in the book. They say that Euron had the idea to sail out into the sea, then sweep back in toward the shield islands. This was specifically done to avoid being seen traveling south along the coast by Lannisport or whoever else.